It is known to use exhaust gas oxygen sensors positioned both upstream and downstream of a catalytic converter to provide air/fuel control of the engine. An example of such usage is found in U.S. Pat. No. 5,115,639.
Various approaches are also known to use the outputs of the upstream and downstream exhaust gas oxygen sensors to provide an indication of catalytic converter efficiency. In one such system, an increase in the amplitude output of the downstream sensor as compared to the upstream sensor provides an indication of converter degradation. In another type of system, the switching frequency of the sensor outputs are compared during steady-state engine operations under a particular combination of engine speed and load.
The inventor herein has recognized numerous problems with the above approaches. For example, amplitude comparison has been found to suffer because exhaust gases from each of the combustion chambers may not be fully mixed in the exhaust manifold. Consequently, there may be variations in sensor amplitude which are caused by lack of exhaust gas mixing and not caused by catalytic degradation. With respect to those systems examining sensor switching frequencies, the inventor herein has discovered at least two significant problems. If the air/fuel feedback loop is in a transient state during the test such that combustion is on average either rich or lean of stoichiometry, there may be insufficient switching of the sensors to provide any reasonable indication of converter efficiency. Further, such systems do not examine converter efficiency over the full range of possible exhaust gas flow rates. Accordingly, inaccurate indications of converter efficiency may result.